Take a look at the body of a 100-meter sprinter and then that of a marathon runner. They look quite different. Are the training methodologies of the two runners, which are at opposite ends of the spectrum, responsible for the difference in body type? Likely not. As it happens, genetics has a large influence over how individuals respond to particular exercise intensities both in terms of the degree of improvement, as well as changes in the size of the affected muscle mass.

At the end of the day, genetics will largely determine how a runner responds to a particular training load and more specifically, what race distance they are most suited for. This of course doesn’t mean that a runner, who is best suited to 800 meters, can’t successfully complete a marathon. It just means that with correct training, they will likely be more successful at 800 meters versus a marathon. Let’s dig into this a little more.

Types of muscle fiber

There are three main muscle fiber types:

1. Type I (slow twitch)
2. Type II A (intermediate twitch)
3. Type II B or IIX (fast twitch)

Type I muscle fibers are the smallest of the three and are characterized by their ability to fire repeatedly with minimal fatigue. However, they don’t contract with the same amount of force as the other two fiber types. In other words, this muscle fiber type has the greatest endurance characteristics.

Type II B fibers are the largest muscle fiber type and contract with the greatest force, but lack the ability to fire repeatedly without fatiguing. Therefore, this fiber type is best suited to short, explosive efforts — such as sprinting. 

Type II A muscle fibers are a hybrid of type I and II B fibers. While technically a fast twitch fiber, it has midrange endurance and contractile capabilities, and hence, the intermediate twitch designation. 

Muscle fiber recruitment

In order to understand how muscles respond to the demands placed on them, it is important to know how muscle fibers are recruited or engaged.

When the intensity of an activity progresses from low or moderate to high, there is a hierarchy as to which types of muscle fibers are recruited. This is called the Henneman’s Size Principle. 

A muscle fiber cannot partially contract, or more specifically, a muscle fiber cannot contract with varying force depending on the effort required. A muscle fiber contracts either 100% or not at all. Therefore, the degree of muscle contraction is based on the number of muscle fibers recruited to perform an action, not the degree of contraction per muscle fiber.

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How to change your muscle fiber type?

Whether or not muscle fibers can change their type has long been debated, and it continues till date. In rare cases, muscles have demonstrated the ability to convert from Type 1 to Type 2 when the muscle was substantially deconditioned, like in the case of an injury. There has, however, not been much evidence to support the theory that muscles can change from Type I to Type II through changes in training routines, although the lack of verification might be due to insufficient research in this particular area.

Regardless of a muscle fiber’s ability to change type, people can always improve their performance, even if it is at odds with their genetic muscle fiber type composition. For example, let us say that you have predominantly fast-twitch muscle fibers and want to compete in an ultramarathon. Through the development and application of the proper training regimen, you can become very proficient despite your fast-twitch genetic makeup to the extent that you can perform quite well in distance running events. 

Techniques for training fast twitch and slow twitch fibers

As noted earlier in the muscle recruitment section, if a runner is looking to focus on training their slow twitch fibers, running at a relatively easy intensity is likely the correct method. Conversely, and due to the Henneman’s Size Principle, if a runner is looking to stimulate their fast twitch fibers, fast and explosive-type exercises such as sprinting and explosive jumping (termed: plyometrics) would likely be the recommended training strategy. 

How does your fiber type affect your performance as a runner?

As discussed previously, just because a runner is targeting a race distance that is at odds with their muscle physiology (primary muscle fiber type), it does not mean that they can’t perform well at that distance. All it means is that from purely a genetic standpoint, they are physiologically optimized to perform best at a particular distance/ intensity. 

Also read: DOMS: What to Know About Delayed Onset Muscle Soreness

For example, if an individual is looking to improve their speed such as sprinting, performing explosive exercises such as vertical jumps and short intervals of 100 meters or less would be advised. Conversely, if an individual is looking to gain endurance, dedicating two to three days per week for easy, long runs that progressively increase in distance based on their current training volume, would likely be a good strategy.